Stereoscopic microscopes of the prior art provide surgeons with a magnified view of a surgical area and have improved the efficacy of surgery. Such microscopes are used in surgical operations by neurosurgeons, otolaryngologists, as well as ophthalmologists. Today, so called micro surgery, which uses a microscope for surgery, is advanced and precise. The surgical microscope is designed so that it may provide images of the surgical area from various directions, and surgery is performed under the observation of two persons, an observer who has primary responsibility for the surgery (hereinafter referred to as the first observer) and an observer (hereinafter referred to as the second observer) who has the responsibility of supporting the first observer in order to improve the safety of the operation.
Many surgical microscopes of the prior art are limited in that the azimuthal directions that the first and the second observer peer into the surgical microscope must differ by 90 degrees or 180 degrees. This sometimes is inconvenient in that, at times, only one observer is able to actually observe the operation site, depending on the direction of the optical axis of the objective lens of the surgical microscope relative to the surgical area. FIG. 17(a) shows an example of a first observer 1 and a second observer 2 that observe a surgical area 4 from azimuthal directions that differ by 180 degrees. The optical axis of the objective lens of the surgical microscope 3 in this case is vertical. FIG. 17(b), on the other hand, shows an example where, using a similar surgical microscope 5 as that in FIG. 17(a), the first observer 7 is able to observe the surgical area 6 with the optical axis of the objective lens tilted from the vertical position. However, in this instance, it is difficult for the second observer 8 to comfortably peer into the microscope eyepiece. This problem occurs, for example, in the microscope shown in FIG. 1 of Japanese Examined Patent Publication S47-41473 and in the microscope shown in FIGS. 2-4 of Japanese Examined Utility Model Publication S55-39364. For most prior art surgical microscopes, the difference in azimuthal angles α between a primary observer and a secondary observer who is positioned on either side of the primary observer, is 90 degrees, as indicated in FIG. 1 of the present application. On the other hand, for a secondary observer who faces the primary observer, the difference in azimuthal angles α is 180 degrees.
The value of α is variable for the microscope disclosed in FIG. 1 of Japanese Laid-Open Patent Application H10-5244. The surgical microscope disclosed therein is shown in FIG. 18 of the present application. Because an optical path splitting means 11 is provided between the objective optical system 9 and the first observation device 10 used by the first observer, the microscope's size must be increased and the distance between the bottom surface 12 of the surgical microscope and the eyepiece lens 13 is increased, thereby decreasing the distance from the bottom surface 12 of the surgical microscope to the surgical area 14 (hereinafter termed the ‘working distance’).
For the microscope shown in FIG. 18, the second observation device 15 that provides observation images to the second observer is arranged directly below the first observation device 10. This causes both the space 19 (illustrated in FIG. 19, wherein the microscope is labeled 16) that is located below the eye level 18 of the first observer 17, when the second observer is positioned opposite the first observer to be narrowed. Referring to FIG. 20, it also causes the space 21 to the right of the first observer 23 when the second observer is to the right of the first observer, to be narrowed. This causes a problem in that a treatment tool 24 (FIG. 20) held by the first observer 23 is more likely to come into contact with the second observation device 22 of the surgical microscope 20, thereby causing an inconvenience during surgery.